977 ISSN 1070-4272, Russian Journal of Applied Chemistry, 2017, Vol. 90, No. 6, pp. 977−992. © Pleiades Publishing, Ltd., 2017. VARIOUS TECHNOLOGICAL PROCESSES Empirical Modeling and CCD-based RSM Optimization of Cd(II) Adsorption from Aqueous Solution on Clinoptilolite and Bentonite 1 Negar Kashi a , Narges Elmi Fard b , and Reza Fazaeli c * a Department of Chemical Engineering, Faculty of Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran e-mail: negarkashi@yahoo.com b Young Researchers and Elite Club, South Tehran Branch, Islamic Azad University, Tehran, Iran e-mail: nargeselmi@yahoo.com c Department of Chemical Engineering, Faculty of Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran *e-mail: r_fazaeli@azad.ac.ir Received June 22, 2017 Abstract—In the present study, adsorption of Cd(II) ion from an aqueous solution on natural clinoptilolite (NC), natural bentonite (NB), modiﬁed clinoptilolite (MC) and modiﬁed bentonite (MB) were investigated. Response surface methodology (RSM) and central composite design (CCD) were used for NC and NB in order to inves- tigate the effect of pH and contact time on adsorption efﬁciency and process optimization. Based on statistical analysis, the Cd(II) adsorption model was highly signiﬁcant, with very low p-values. pH’s 5.35 and 3.89, and contact times 20.49 and 16.27 h, with adsorption efﬁciency 94.86% and 87.42%, were found to be optimum for Cd(II) ion adsorption on NC and NB, respectively. The experimental data was ﬁtted to adsorption isotherm models, indicating the monolayer sorption of Cd(II). The Jossesns, Unilan, Baudu, and Freundlich models were selected as the best models, with correlation coefﬁcients of 0.8973, 0.9930, 0.9267, and 0.9723, and with lowest error for NC, NB, MC, and MB, respectively, with negligible differences compared to the experimental values. The results showed that the NC adsorption efﬁciency is higher than NB in both cases. DOI: 10.1134/S1070427217060210 1 The text was submitted by the authors in English. INTRODUCTION In most parts of the world, attention is being paid to heavy metals, in various chemical and physical forms and in different concentrations, on account of environmental pollution. These heavy metals are discharged into the environment as industrial waste, which endangers human health and other organisms. Heavy metals, such as cad- mium, zinc, copper, lead, nickel, arsenic, etc., accumulate in the human body and organisms, and are rather danger- ous [1, 2]. Cd(II) is a toxic heavy metal in the environ- ment that is carcinogenic, and its toxicity causes damage to the central nervous system, psychological disorders and pulmonary insufﬁciency, and has negative effects on bones, liver and blood. Therefore, it is essential to remove cadmium from wastewater before it is discharged [3–5]. Various methods have been reported for the removal of heavy metal from wastewater and water samples such as ion exchange, ionic-imprinted polymers, reverse os- mosis, electro-coagulation, complexation ultraﬁltration, ultraﬁltration membranes, nanoﬁltration, electroﬂoc- culation ﬁltration, hybridization electrodialysis/electro- deionization, copolymeric adsorbents, precipitation, hollow ﬁber, coagulation, solvent extraction, biosorption and adsorption have been used and developed to remove heavy metals from wastewater [6]. Among these methods, the adsorption method has received much attention due to its high efﬁciency and low cost. An adsorbent should be eco-friendly, cost-effective and efﬁcient for a wide range of concentrations of different heavy metals [7]. In a typical adsorption process, a porous medium with a large